The present invention relates to a design method of devolatilizers, especially to a design method of batch falling strand devolatilizers that is applied to a scale-up design of the batch falling strand devolatilizers so as to achieve preset high devolatilization efficiency of the batch falling strand devolatilizer.
US researchers Biesenberger, J. A. (Devolatilization of Polymers, Hanser Publishers, N.Y., 1983) and Biesenberger, J. A. and Sebastian, D. H. (Principles of Polymerization Engineering, Wiley-Interscience, NY., p.573-659, 1983) have reviewed principles of various devolatilizers applied in polymerization engineering industry. The later one has discussed the batch falling strand devolatilizer and has focused on microscopic theories such as fluid mechanics, heat transfer and mass transfer. However, the batch falling strand devolatilizer according to the present invention emphasizes macroscopic parameters of engineering design such as sizing of devolatilizers, polymer viscosity, recycles of batch operation and whole process efficiency.
Chinese scientists Li, G. et al. (Devolatilization of hydroxy-terminated polydimethylsiloxane in single-screw extruder, Hecheng Xiangjiao Gongye, 27(4), p. 205-208 (in Chinese), 2004, ISSN:1000-1255, Chemical Abstract 142:115180) have studied parameters related to volatiles of polydimethylsiloxane (PDMS) with end hydroxyl group and found that temperature and vacuum level are the most important factors. They also got relationships among final amount of volatiles, flow rate and rotating speed. Yet the devolatilizer is a single-screw extruder that is different from the falling strand devolatilizer according to the present invention. Moreover, Biesenberger, J. A. (Polymer devolatilization: equipment theory, 37.sup.th Society of Plastic Engineer, ANTEC, New Orleans, p.972-978, 1979), Latinen, G. A.(Devolatilization and polycondensation process, ACS Series No. 34, Advances in Chemistry Series, American Chemical Society, p.235-246, 1962) and Powell, K. G. (Devolatilisation of PDMS gums: a performance comparison of co- and counter-rotating twin-screw extruders, Antec '94 Conference Proceedings, San Francisco, Calif, 1-5, May, 1994, Vol. I, p.234-8.012) have discussed design model of screw devolatilization extruders.
Recently, German researcher Nonnenmacher, S. (Numerical and experimental investigation of volatilization in statistical devolatilization apparatus, Fortschritt-Berichte VDI, Reihe 3: Verfahrenstechnik, 793, i-xviii, 1-158 (in German), 2003, VDI Verlag Gmbh, ISSN:0178-9503, Chemical Abstract 139:396407) has discussed theories and experiments of falling strand devolatilizers and flash evaporators. Both devolatilizers have no mechanical movement parts but use gaseous stripping agents. The design principles are different from the present invention.
In Silicones, Process Economics Program, Report No. 160, p.283, June, 1983, SRI International, Menlo Park, Calif. 94025 published by the Stanford Research Institute, Scheeline, H. W. and Chandwani, D. disclosed a flow chart of chemical equipments that shows thin-film evaporators are applied to silicon polymer manufacturing industry currently. The device is different from the batch falling strand devolatilizer according to the present invention.
Refer to U.S. Pat. No. 4,096,160—Continuous devolatilization of silanol-terminated silicone polymer, a devolatilization process is disclosed. An improved process is provided for preparing a linear diorganopolysiloxane fluid having terminal hydroxy groups, which is substantially free of cyclic polysiloxanes. In the process, a linear diorganopolysiloxane fluid containing lower boiling cyclic polysiloxanes is mixed with steam into an evacuated, tortuous, confined passageway to form a turbulent mixture that passed through the passageway. The vaporized mixture of steam and cyclics and the linear diorganopolysiloxane substantially free of cyclics are removed from the passageway, and the vaporized mixture of steam and cyclics is separated from the cyclic-free diorganopolysiloxane fluid. The system is different from the device of the present invention.
Refer to U.S. Pat. No. 4,430,461—Method of removing volatiles in the preparation of silicone compositions, a method of removing volatiles in the mixing of a high-temperature-vulcanizing silicone rubber composition is disclosed. The high-temperature-vulcanizing silicone rubber has a viscosity ranging from 1,000,000 to 300,000,000 centipoise so that the volatiles are removed by aspirator means connected to a Banbury-type mixing vessel for producing a devolatilized mixture. The design principles of the device and the present invention are far more different from each other. Furthermore, U.S. Pat. No. 3,960,802—Process for the manufacture of a one-component room-temperature vulcanizable silicone composition, and U.S. Pat. No. 4,528,324—Process for producing RTV silicone rubber compositions using a devolatilizing extruder both use devolatilizing extruders to perform mixing and devolatilization of a formulated room temperature vulcanizable silicone rubber composition. But the present invention is a concern of falling strand devolatilizer without any mechanical rotation.
Refer to U.S. Pat. No. 3,987,235—Devolatilization of alkenyl aromatic polymers, alkenyl aromatic polymers such as styrene polymers are devolatilized in a molten condition by the introduction of methanol and the volatiles removed under vacuum. U.S. Pat. No. 4,124,658—Devolatilization of styrene polymers with sulfonylhydrazides, discloses residual monomer from styrene-based polymers is removed by adding a sulfonylhydrazide to the polymer and heating the mixture to a temperature above the decomposition temperature of the sulfonylhydrazide used. In U.S. Pat. No. 5,861,474, contaminants in thermoplastic polymer are removed by introducing stripping agents such as N2, C2H4, CH4, CO2 while U.S. Pat. No. 6,124,426 discloses a devolatilizing process by adding a blowing agents such as water, acetone, or methanol to a styrene/acrylonitrile copolymer (SAN)-containing volatile materials. In U.S. Pat. No. 6,211,331, stripping agents such as water, methanol or a solution of carbon dioxide in water are used for devolatilisation of a thermoplastic polymer, and U.S. Pat. No. 6,353,088 discloses a foaming agent such as water or alcohol that is immiscible with the polymers for removing volatile matters from aromatic vinyl polymers. Similarly, U.S. Pat. No. 6,410,683 shows that the polymer is mixed with stripping agent for the removal of impurities from a thermoplastic polymer.
In U.S. Pat. No. 6,740,691—Removal of volatile organic compounds from polymer dispersions, by introducing oxidisable sulphur compounds, free radical intiator and steam, emulsion and suspension polymerization is performed for removal of volatile organic compounds. In Taiwanese patent No. 402,612, a method to remove volatile materials from thermoplastic polymer by using water, methanol and acetone is disclosed. All above-mentioned patents improves the operation efficiency of devolatilization by introducing additives. The design of above patents and the design of the present invention are based on different principles.
U.S. Pat. No. 4,383,972 and U.S. Pat. No. 4,439,601 disclose design of a multiple stage flash evaporator. By lower pressure (or higher vacuum level) of lower-stream flash evaporator than pressure of upper-stream flash evaporator, continuous processes are performed for removing volatiles in polymers. U.S. Pat. No. 4,537,954 discloses removing volatile components continuously from the polymerization fluid composition in three stages. The lower stream the device is, the higher temperature and the vacuum level they are. In the third stage, devolatilization is carried out in the presence of a foaming agent. By control of temperature and pressure in each stage, volatile components are removed continuously. The design fundamental is different from that of the present invention.
As to U.S. Pat. No. 4,452,977—Process for the preparation of polymer melts which are substantially free of volatile components, U.S. Pat. No. 4,578,455—Process and apparatus for removing volatile constituents from polymer melts or pastes, U.S. Pat. No. 6,833,096—Method for removing water and other volatile components from polymer powders, and Taiwanese patent No. 307,709—Extrusion method and device thereof for removal of volatiles from solid-state resin, all reveal extruders for continuous devolatilization operations. The system for devolatiolization of above patents is different from the present invention.
U.S. Pat. No. 4,744,957 discloses an apparatus having a polymer discharge device provided with heating means and adapted to discharge a polymer charged therein in the form of a strand or film and so forth for removing the volatile matters from the polymers, is disclosed. However, this is a continuous flow process without mention a design method. U.S. Pat. No. 5,024,728 discloses similar devolatilization apparatus of the present invention that removes volatile constituents of liquid streams by vacuum and high temperature. The apparatus includes a vertical multi-tube heat exchanger disposed between devolatilizer and raw material and a static mixer is disposed inside the system. The system is for continuous processes and the design method is not mentioned. Therefore, the system is different from the present invention.
U.S. Pat. No. 4,865,689—Method and apparatus for evaporating the volatile components of a polymer and U.S. Pat. No. 7,060,788—Process for stripping monomers and other volatile constituents from polymer melts respectively discloses a thin-film evaporator without and with rotating devices. The machines are different from the present invention.
Refer to U.S. Pat. No. 4,954,303—Apparatus and process for devolatilization of high viscosity polymers, the apparatus includes a vacuum chamber atop of which the polymer is introduced, with a low shear mixer located in the vacuum chamber and a pumping device for agitating polymeric material within said vacuum zone and removing the devolatilized polymer. The apparatus is different from the system of the present invention.
As to U.S. Pat. No. 5,084,134—Process for the devolatilization of polymer solutions, by control of the ratio between thermal exchange surface and the flow per hour of the solution, flow speed in the channel, and residence time in the channel, thermosensitive polymer is purified. Such hardware design is different from design of the present invention. Refer to U.S. Pat. No. 5,453,158—Polymer devolatilizer, a flat plate heat exchanger is used to remove volatiles from polymers and this is also quite different from process and hardware design of the present invention.
U.S. Pat. No. 6,485,607—Methods for removing volatile components from polymer solutions, U.S. Pat. No. 6,627,040—Device and method for removing volatile components from polymer solutions, and Taiwanese patent No. 552,284—Method for removing volatiles from polymer solution disclose devices made from metallic material having a low iron content. Such design and the present invention are based on different principles.
After polymer synthesis, non-reactive monomers, solvents and oligomers are removed from the target polymers. Now advantages and disadvantages of prior arts and the present invention are compared as followings:    1. Most of prior arts perform devolatilization by devices such as thin-film evaporators or devolatilizing extruders. These devices include mechanical rotating components. Equipment investment cost as well as operating cost is far more higher than falling strand devolatilizers without mechanical rotating components according to the present invention.    2. Water, hydrocarbons and inert gas such as nitrogen, carbon dioxide etc. are often used as entraining agent for devolatilization in prior arts. Thus efficiency of volatile condenser is reduced and loading of the vacuum system is increased. Therefore, the cost is raised. As to the present invention, there is no need to use entraining agent.    3. Most of prior arts are continuous processes that meet requirements of mass production. For small and medium enterprise, it cost a lot to purchase such equipments. The flexibility of the continuous process may be not as good as that of the operation in a batch mode or in a quasi-continuous processing mode. Yet the present invention is applicable to a batch processing or quasi-continuous process formed by continuous batch recycles.    4. In prior arts, different types of devolatilizers are applied to polymers with different viscosities. For example, in U.S. Pat. No. 5,084,134, when the viscosity of polymer is under 106 cp, it is suggested to use thin-film evaporators for devolatilization while devolatilizing extruders are applied to polymer with viscosity over 106 cp. But pipelines of the devolatilizing extruder revealed in U.S. Pat. No. 4,954,303 are often stopped by accumulated polymers and the system must be shut down for periodically cleaning. Moreover, high shear effect of extruder may lead to reduction of molecular weight or particle size of polymers and this cause quality damage. The present invention do not use devolatilizing extruder.    5. Most of prior arts are related to designs of hardware, equipment material or operations. None of them provides software principle of scale-up design. There are three factors-fluid mechanics, heat transfer and mass transfer related to design of devolatilizer while fluid mechanics and mass transfer play key roles. In prior arts, relationship among the factors hasn't been established so that the scale-up design takes a lot of try and error tests. However, the present invention provides a design method of batch falling strand devolatilizers that achieve devolatilization requirements of products by a time for space approach.